Atomic absorption photometer

ABSTRACT

In an atomic absorption photometer, depending on a fact whether a background correction is carried out or not, a pattern of a pulse lighting of light sources and a timing of sampling a light receiving signal are changed. Namely, at the time of a background correction measurement, one cycle is divided into three periods, that is, a period of lighting HCL, a period of lighting D 2 L, and an off period, and at the time of a measurement without a background correction, one cycle is divided into the period of lighting HCL and the off period. Accordingly, at the time of the measurement without the background correction, the period of lighting HCL and the off period become longer, and signal-to-noise ratio of the light receiving signal is improved. Accordingly, an absorbance with high accuracy can be calculated.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an atomic absorption photometer, andmore particularly, to an atomic absorption photometer which carries outa correction in a background absorption using a continuous spectrumsource, such as a deuterium lamp.

When atomic absorption photometers are roughly classified, there are aflame method and a furnace method. In the flame method, a sample issprayed in a flammable gas, and by burning the mixed gas on a burnerhead, the sample is heated at a high temperature to be atomized. Then, ameasuring beam is allowed to pass through the atomic vapor to therebymeasure an absorbance. In the furnace method, a sample is held in agraphite tube (heating tube), and by heating the tube, the sample isheated at a high temperature to be atomized. Then, the measuring beam isallowed to pass through the atomic vapor to thereby measure theabsorbance.

In these atomic absorption analyses, for example, in case a large amountof salts and the like are mixed in the sample, they are not completelydissociated even at the high temperature, and may cause the scatteringand absorption with respect to the measuring beam from a light source.Also, in the flame method, if a flame temperature is low, an absorptionby a molecular type in the flame may occur. These phenomena are called abackground absorption. In case of using a light source which emits abright line spectrum, such as a hollow cathode lamp (HCL) which is oftenused as the light source in general, an absorbance by the atomicabsorption of the specified element is added to an absorbance by thebackground absorption described above. Thus, it is difficult to obtainthe accurate absorbance.

Therefore, there has been conventionally conducted a correction called adeuterium lamp background correction, which is also called a continuousspectrum source system. Namely, other than the light source which emitsthe bright line spectrum, an absorbance with respect to a measuring beamfrom a light source which emits a continuous spectrum (normally, thedeuterium lamp is used) is measured at the same time. In case of usingthe light source which emits the continuous spectrum, since a wavelengthbandwidth is extremely broad, the absorbance due to the atomicabsorption of the specified element is small such that it can besubstantially ignored, and it can be considered that only the absorbanceby the background absorption is measured. Therefore, by calculating adifference between these absorbances, an effect by the backgroundabsorption is eliminated, and only the absorbance by the specifiedelement can be obtained. Incidentally, throughout the specification ofthe present invention, in case the background correction is merelyreferred to, it means the background correction using the deuteriumlamp.

Also, as an error factor other than the background absorption, theremight be an effect of an extraneous light or outside light. Namely, theflame in the flame method and the graphite tube in the furnace methodemit lights by themselves, and since a sample chamber becomes hot in theatomic absorption photometer, the sample chamber is often in an opencondition, so that the light from outside may be introduced into aspectroscope. Although these lights are irrelevant to the atomicabsorption, in case these lights have components of wavelengths whichare observed, they cause enormous errors in the measurement. Thus, it isnecessary to eliminate these lights.

In order to eliminate the effect of the background correction and theoutside light, in the conventional atomic absorption photometer, lightsor pulse lights of the light sources are controlled so that threeperiods, that is, a period of light of only the hollow cathode lamp, aperiod of light of only the deuterium lamp, and an off period of tuningof f both lamps, are provided. Then, during the off period, a signal zdue to the outside light or dark current of a photodetector itself isobtained, and by subtracting the signal z respectively from a lightreceiving signal h obtained at the photodetector during the period oflight of only the hollow cathode lamp, and from a light receiving signald obtained at the photodetector during the period of light of only thedeuterium lamp, light receiving signals h′ and d′, in which the effectof the outside light is eliminated, are obtained. Furthermore,absorbances are respectively calculated from the light receiving signalsh′ and d′, and by finding the difference between these absorbances, thebackground correction is carried out.

In the atomic absorption analysis, the background correction is notalways carried out, and for example, in case it is known in advance thatthe sample does not contain substances which cause the backgroundabsorption, the background correction is not required. Also, in case ameasurement object wavelength is in a wavelength region (more than 425nm) in which an emission spectrum of the deuterium lamp is very weak,the background correction is not useful, so that it is not required. Atthe time of the measurement without the background correction describedabove, in the conventional atomic absorption photometer, there iscarried out only a control such that the deuterium lamp which isunnecessary as the measuring beam is left to be turned off, and it hasnot been considered to positively improve a signal-to-noise ratio of thesignal at the time of measurement without the background correction.

Accordingly, the present invention has been made in view of theforegoing, and an object of the invention is to provide an atomicabsorption photometer, in which a signal-to-noise ratio of a lightreceiving signal at the time of measurement without backgroundcorrection is improved, so that the absorbance can be calculated withhigh accuracy.

Further objects and advantages of the invention will be apparent fromthe following description of the invention.

SUMMARY OF THE INVENTION

To achieve the aforementioned object, the present invention provides anatomic absorption photometer, in which light emitted from a light sourceis allowed to pass through an atomization section, and the light passingthrough the atomization section is introduced into a photodetector orfirst photodetector to measure an intensity thereof, so that anabsorbance at the atomization section is calculated from a signal fromthe photodetector. The atomic absorption photometer as stated abovecomprises a first light source for emitting a bright line spectrumlight; a second light source for emitting a continuous spectrum light;and measurement controlling means for dividing a predeterminedmeasurement period into three periods formed of a first measurementperiod by a light emitted from the first light source, a secondmeasurement period by a light emitted from the second light source, anda third measurement period in a condition that neither of emitted lightsfrom the first and second light sources are emitted on the photodetectorin case a background correction is carried out. Also, the measurementcontrolling means distributes the second measurement period to one orboth of the first measurement period and the third measurement period incase the background correction is not carried out.

At the time of the measurement without the background correction, themeasurement by the light emitted from the second light (for example,deuterium lamp) is not required. Thus, in this case, the measurementcontrolling means distributes the second measurement period, which isassigned to the measurement by the emitted light from the second lightsource in case of the measurement with the background correction, to oneor both of the first measurement period by the emitted light from thefirst light source and the third measurement period in the conditionthat neither of the emitted lights from both the light sources areemitted. Therefore, at least one of the first measurement period and thethird measurement period becomes longer than that at the time of themeasurement with the background correction.

In the photodetector which receives the measuring beam or light, as aperiod of time for receiving the measuring beam becomes longer, thelight receiving signals are integrated to thereby improve thesignal-to-noise ratio. If the signal-to-noise ratio of the lightreceiving signal is improved, the accuracy of the absorbance calculatedbased on the light receiving signal is improved. Thus, according to theatomic absorption photometer of the invention, especially in case themeasurement without the background correction is carried out, thesignal-to-noise ratio of the light receiving signal is improved ascompared with the conventional atomic absorption photometer, resultingin increasing the accuracy of the absorbance.

Also, in the atomic absorption photometer according to the presentinvention, there may be provided filtering means for reducing aradio-frequency region of the light receiving signal by thephotodetector, and the cut-off frequency of the filtering means can bechanged in accordance with the existence of the background correction.

The filtering means is provided for removing a high frequency noisecontained in the light receiving signal, and it is preferable to set ahigh region cut-off frequency of the filtering means as low as possiblewithin a range of not losing the frequency component as the object.Normally, since the cut-off frequency is determined by the repeatingcycle of the first measurement period, the second measurement period,and the third measurement period, in case the second measurement periodis distributed to the first and third measurement periods as describedabove, the cut-off frequency can be lowered. According to thisstructure, since the high frequency noise is further reduced in case thebackground correction is not carried out, the signal-to-noise ratio ofthe light receiving signal can be further improved.

Also, the atomic absorption photometer according to the presentinvention may be further provided with a second photodetector includinga detection sensitivity in a long wavelength region, and an optical pathswitching means for selectively introducing the light passing throughthe atomization section into one of the photodetector and the secondphotodetector. In this structure, in case the background correction isnot carried out, the optical path switching means switches the opticalpath such that the light is introduced into the second photodetector.

Here, for example, the first photodetector is a photomultiplier, and thesecond photodetector is a photodiode. Although the photodiode has adisadvantage in the response speed, in case the second measurementperiod is distributed to the first and third measurement periods asdescribed above, the first and third measurement periods become longer,and the slowness of the response speed does not cause so much problem.In general, since the background correction is not carried out in mostof the cases when the wavelength as the measurement object is in a longwavelength region outside a range of the wavelengths of the emittedlight of the deuterium lamp, it is possible to sufficiently use thedetector, such as photodiode, which does not have a detectionsensitivity in a short wavelength region. According to this structure,since the detection of the light in the long wavelength region can beassigned to the second photodetector, the photodetector having thedetection sensitivity in the short wavelength region is sufficient.Among the photomultipliers, the photomultiplier having thischaracteristic is considerably cheaper than a photomultiplier having ahigh detection sensitivity also in the long wavelength region, and thephotodiode is much cheaper. Therefore, according to this structure, thecost of the atomic absorption photometer can be further reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a structure of a main section of anatomic absorption photometer according to a first embodiment of theinvention;

FIG. 2 is a flow chart showing characteristic operations in the atomicabsorption photometer of the first embodiment of the invention;

FIGS. 3(a) to 3(d) are timing charts for explaining the operations ofthe atomic absorption photometer;

FIG. 4 is a block diagram showing a structure of a main section of anatomic absorption photometer according to a second embodiment of theinvention; and

FIG. 5 is a graph schematically showing detection sensitivitycharacteristics of elements used as a photodetector in the atomicabsorption photometer according to the second embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, an atomic absorption photometer as a first embodiment of theinvention will be explained with reference to the accompanied drawings.In this example, the present invention is applied to a furnace typeatomic absorption photometer. However, as clearly understood from theexplanation described later, since a difference in the atomizationsection is not relevant to the present invention, it is needless to saythat the present invention can be applied to the flame type atomicabsorption photometer.

FIG. 1 is a block diagram showing a structure of a main section of theatomic absorption photometer of the first embodiment. A light sourcesection 10 includes a hollow cathode lamp (HCL) 11, a deuterium lamp(D2L) 12, and a half mirror 13. A light or beam including a bright linespectrum emitted from the HCL 11 becomes HCL luminous flux Lh, and HCLluminous flux Lh is received on the half mirror 13. On the other hand, alight or beam emitted from the D2L 12 becomes D2L luminous flux Ld,which is received on the half mirror. At the half mirror 13, the HCLluminous flux Lh and D2L luminous flux Ld are combined into a singleluminous flux Lm. However, the HCL 11 and D2L 12 are controlled to besubjected to pulse light by a light source driving section 26 under acontrol by a processing and controlling section 22, described later, andthe luminous flux Lm results from a time division multiplexing of theHCL luminous flux Lh and D2L luminous flux Ld including off time. Theluminous flux Lm passes through an inside of a graphite tube 15 of anatomization section 14 to be introduced into a monochromator 16, andlight with a predetermined wavelength is taken out by the monochromator16 and sent to a photodetector 17.

Incidentally, although not shown in the drawing, adequate condensingoptical systems are provided respectively between the light sourcesection 10 and the atomization section 14, and between the atomizationsection 14 and the monochromator 16, and these condensing opticalsystems condense the luminous fluxes adequately to thereby introduce thecondensed luminous flux to the next stage.

Electric signal obtained by being photoelectrically converted at thephotodetector 17 are amplified at an amplifier 18, and a radio-frequencynoise is removed at a low pass filter (LPF) 19. Then, the electricsignals are separated into the respective signals corresponding to lightof the HCL, light of the D2L, and off condition of both lamps at asynchronous circuit 20, and are further converted into digital signalsat an analog-to-digital converter 21 to be inputted into the processingand controlling section 22. The processing and controlling section 22 ismainly formed of a computer including a CPU, and the processing andcontrolling section 22 carries out various operation processes andoutputs control signals for controlling operations of the respectivesections described above. Also, the processing and controlling section22 is connected to a memory 23, an operating section 24, such as akeyboard, and a display section 25, such as CRT (cathode ray tube)display.

At the time of a quantitative analysis of the sample, a large current isallowed to flow from a current source, not shown, through the graphitetube 15, so that the graphite tube 15 is heated. Then, a sample liquidis introduced into the graphite tube 15 from a sample injection portprovided at an upper section of the graphite tube 15, and the sample isatomized therein. As described above, in the light passing through thegraphite tube 15, the light having the wavelength peculiar to theelement contained in the sample is intensely absorbed. The processingand controlling section 22 calculates a ratio between a light receivingintensity without being subjected to the absorption and a lightreceiving intensity subjected to the absorption, and the sample isquantified from the absorbance obtained as described above.

Next, characteristic processing operations in the atomic absorptionphotometer according to the first embodiment will be explained inaccordance with a flow chart shown in FIG. 2. Also, FIGS. 3(a) to 3(d)are timing charts for explaining the processing operations.

When an operator sets various parameters from the operating section 24prior to the measurement, as one of the parameters, the operator selectswhether the background correction is required or not. When themeasurement is started, the processing and controlling section 22determines whether a background correction is required or not (step S1).In case of the background correction measurement, processes in steps S2through S5 are sequentially carried out, and in case of the measurementwithout the background correction, processes in steps S6 through S9 aresequentially carried out.

Namely, in case of the background correction measurement, there is setan operation mode such that a measurement period of one cycle is dividedinto three periods, that is, a period of lighting HCL, a period oflighting D2L, and an off period of tuning off both light sources (stepS2), and furthermore, it is set to switch a cut-off frequency of the LPF19 to a higher side (step S3). Then, by operating the light sourcedriving section 26 and the synchronous circuit 20 in accordance with theoperation mode described above, the measurement is carried out (stepS4). At this time, the respective operation conditions become as shownin FIG. 3(a), and for example, the D2L 12 is turned off during theperiod of lighting HCL, and the light receiving signal obtained at thephotodetector 17 at this moment is sampled during a period of pulsesignal “1” shown at (b1) in FIG. 3(b), to thereby utilize the same asthe light receiving signal with respect to the emitted light from theHCL 11. The same operations are carried out in the period of lightingD2L and the off period.

Therefore, from the measurement described above, there can be obtainedthe light receiving signal h with respect to the emitted light from theHCL 11, a light receiving signal d with respect to the emitted lightfrom the D2L 12, a light receiving signal z (due to the outside light,dark current or the like) with respect to the off period, so that theabsorbance after conducting the background correction and theelimination of the outside light is calculated from the followingformula (step S5).

Absorbance=F(h−z)−F(d−z)

Here, F(x) is a formula for calculating the absorbance with respect to alight receiving signal x.

On the other hand, in case of the measurement without the backgroundcorrection, there is set an operation mode such that the samemeasurement period of one cycle is divided into two periods, that is,the period of lighting HCL and the off period (step S6), andfurthermore, it is set to switch the cut-off frequency of the LPF 19 toa lower side (step S7). Then, the light source driving section 26 andthe synchronous circuit 20 are operated to carry out the measurement(step S8). The respective operation conditions are shown in FIG. 3(c),and at this time, the period of lighting D2L which is necessary for thebackground correction is not provided, and for that portion, the periodof lighting HCL and the off period become relatively longer. In thisexample, one cycle is substantially equally divided into three at thetime of the background correction measurement, and one cycle issubstantially equally divided into two at the time of the measurementwithout the background correction. Therefore, the period of lighting HCLand the off period at the time of the measurement without backgroundcorrection are respectively about one and a half times longer than thoseat the time of the background correction measurement. Therefore, forthose longer periods, electrons are integrated at the photodetector 17,so that the signal-to-noise ratio of the light receiving signal isimproved.

Also, since it is necessary that the LPF 19 allows signal changecomponents, which correspond to alternation cycles of the respectiveperiods within the period of one cycle described above, to passtherethrough, in the measurement without background correction in whichthe alternation cycle is relatively slow, the cut-off frequency of theLPF 19 can be lowered. Therefore, the high frequency noise contained inthe signal is reduced accordingly, and in view of this aspect, thepresent invention is effective for improving the signal-to-noise ratioof the received signal. Incidentally, for example, in the LPF by an RCcircuit, switching the cut-off frequency can be carried out by astructure of changing a value of R or C by a switch.

From the measurement described above, there can be obtained the lightreceiving signal h with respect to the emitted light from the HCL 11,and the light receiving signal z (due to the dark current or the like)with respect to the off period. Thus, the absorbance after conductingthe elimination of the outside light is calculated by the followingformula (step S9).

Absorbance=F(h−z)

Since the signal-to-noise ratios of the light receiving signals h and zare higher than those in the conventional atomic absorption photometer,the accuracy of the absorbance is improved consequently.

Incidentally, instead of lighting and turning off the HCL 11 and D2L 12,it can be structured that a rotating sector mirror is used toselectively introduce lights into the atomization section, and at thesame time, by utilizing a rotating chopper, the light to be provided onthe photodetector 17 may be intercepted to produce a periodcorresponding to the off period.

Also, although the measurement period of one cycle is divided equallyinto two or three in the aforementioned embodiment, it is not alwaysnecessary to divide the period equally.

Further, although the aforementioned embodiment has the structure of thesingle beam system, the present invention can be applied to a structureof a dual beam system. In this case, the pulse signal which controlson/off turning of the light source is synchronized with an operation ofa mechanism of splitting the luminous flux, such as a chopper mirror,and it may be controlled that the light source is not lighted up duringa period corresponding to a boundary of splitting the luminous flux.

Next, an atomic absorption photometer according to a second embodimentof the invention will be explained with reference to FIG. 4 and FIG. 5.FIG. 4 is a block diagram showing an entire structure of the atomicabsorption photometer according to the second embodiment, and FIG. 5 isa graph schematically showing the detection sensitivity characteristicsof the elements used as photodetectors in the atomic absorptionphotometer of the second embodiment.

In case a wide range in the order of 190 to 900 nm is required to becovered as a measurement wavelength region in the atomic absorptionanalysis, in order to correspond to this wide range by using a singlephotodetector as in the first embodiment, it is necessary to use aphotomultiplier having a characteristic shown by a broken line in FIG.5. In general, this kind of photomultiplier is considerably expensive.On the contrary, in a relatively inexpensive photomultiplier, althoughenough detection sensitivities can be obtained in a short wavelengthregion, since the detection sensitivity is drastically lowered in thelong wavelength region (more than 650 nm), it is difficult to cover theentire wavelength region. On the other hand, there is a photodiode as afurther inexpensive detector, and even though the photodiode is inferiorto a high performance photomultiplier in the long wavelength region inthe order of 400 to 1000 nm, the photodiode has enough detectionsensitivities in the long wavelength region. However, in the photodiode,if it is tried to increase gains especially, the response speed of thephotodiode becomes slower, so that it can not follow the high speedon/off cycle.

In the atomic absorption photometer of the second embodiment, byutilizing that the period of lighting the HCL 11 becomes substantiallylonger at the time of the measurement without the background correctionas described above, the inexpensive photomultiplier and the photodiodecan be adopted as the photodetector. Namely, in FIG. 4, thephotodetector 17 is, for example, a photomultiplier which is inexpensiveand has high detection sensitivities only in the short wavelength regionas shown by a solid line in FIG. 5. Before the photodetector 17, thereis provided a mirror 30 for switching the optical path, which is capableof being freely inserted into the optical path and retreated therefrom,and in case the mirror 30 is inserted into the optical path, the lighttaken out at the spectroscope 16 is introduced into a photodiode 31.Also, an electric signal obtained by being photoelectrically convertedby the photodiode 31 is amplified at an amplifier 32, and selected as anoutput of the amplifier 18 by a turnover switch 33 to be inputted intothe LPF 19.

In this atomic absorption photometer, in case the background correctionmeasurement is carried out, the processing and controlling section 22allows the mirror 30 to be retreated from the optical path, and bybringing the turnover switch 33 to the right in the figure, theprocessing and controlling section 22 allows the output of the amplifier18 to be inputted into the LPF 19. Accordingly, the operations explainedin the first embodiment are achieved. On the other hand, at the time ofthe measurement without the background correction, the mirror 30 isinserted into the optical path, and by bringing the turnover switch 33to the left, the output of the amplifier 32 is inputted into the LPF 19.Accordingly, the measuring beam which has passed through the inside ofthe graphite tube 15 is introduced onto the photodiode 31, and the lightreceiving signal thereof is sent to the LPF 19 via the amplifier 32.

In general, the measurement without the background correction isselected mostly when the measurement object wavelength is a longwavelength region where an emission spectrum of the D2L 12 is extremelyweak (wavelength region more than 425 nm) so that the backgroundcorrection is substantially meaningless. Therefore, there is no problemeven if the photodiode 31, which has almost no detection sensitivity inthe short wavelength region, is used as the detector at the time of themeasurement without the background correction. Needless to say, sincethe measurement in the short wavelength region is expected at the timeof the background correction measurement, there is no problem of usingthe inexpensive photomultiplier which has almost no detectionsensitivity in the long wavelength region.

As described above, in the atomic absorption photometer of the secondembodiment, in accordance with the selection between the backgroundcorrection measurement and the measurement without the backgroundcorrection, the photomultiplier and the photodiode can be properly andalternately used as the photodetector. As described above, thecorresponding wavelength regions of the respective elements can belimited, respectively, and even if costs of these two elements, themirror 30 and other additional parts are added, the cost of the atomicabsorption photometer of the invention can be lower than that of theatomic absorption photometer in which a single, expensive andhigh-performance photomultiplier is used.

Incidentally, the aforementioned embodiments are examples of theinvention, and can be adequately modified within the gist of the presentinvention.

What is claimed is:
 1. An atomic absorption photometer, comprising: alight source formed of a first light source for emitting a bright linespectrum light and a second light source for emitting a continuousspectrum light, an atomization section for allowing a light emitted fromthe light source to pass therethrough to measure a material thereat, afirst photodetector for receiving the light passing through theatomization section, said first photodetector measuring an intensity ofthe light to issue a signal thereof so that an absorbance at theatomization section is calculated, and measurement controlling means forcontrolling the light source and the first photodetector, saidcontrolling means, in case a background correction is carried out,dividing a predetermined measurement period into three periods formed ofa first measurement period measured by a light emitted from the firstlight source, a second measurement period measured by a light emittedfrom the second light source, and a third measurement period measured ina condition that no lights from the first and second light sources areemitted on the photodetector, said measurement controlling meansdistributing the second measurement period to at least one of the firstmeasurement period and the third measurement period in case thebackground correction is not carried out.
 2. An atomic absorptionphotometer according to claim 1, further comprising filtering meanselectrically connected to the photodetector and the controlling meansfor reducing a radio-frequency region of a light receiving signal by thefirst photodetector, said filtering means having a cut-off frequencyvaried in accordance with an existence of the background correction. 3.An atomic absorption photometer according to claim 1, further comprisinga second photodetector having a detection sensitivity in a wavelengthregion different from that of the first photodetector, and optical pathswitching means for selectively introducing the light passing throughthe atomization section into one of the first photodetector and thesecond photodetector, said optical path switching means switching anoptical path such that the light is introduced into the secondphotodetector in case the background correction is not carried out. 4.An atomic absorption photometer according to claim 3, wherein saidsecond photodetector has the detection sensitivity in a wavelengthregion between about 400 and 1000 nm.
 5. An atomic absorption photometeraccording to claim 1, further comprising a spectroscope situated betweenthe first photodetector and the atomization section.
 6. An atomicabsorption photometer according to claim 1, wherein said light sourcefurther includes means to combine the bright line spectrum light fromthe first light source and the continuous spectrum light from secondlight source, said means introducing the lights to the atomizationsection.